1. Field of the Invention
The present invention provides a power transform device and an electronic device, and more particularly, to a power transform device and an electronic device for reaching low acoustic noise.
2. Description of the Prior Art
There are various elements in an electronic device, and each of said elements requires a specific operating voltage. Therefore, in an electronic device, a power transform device is utilized for transforming voltage levels in a raising manner or in a reductive manner, and for stabilizing said transformed voltage levels. In the prior art, a conventional power transform device is manipulated by utilizing a pulse width modulation technique or a pulse frequency modulation technique for supplying transformed power.
Please refer to FIG. 1, which is a diagram of a conventional power transform device 100. The power transform device 100 comprises a capacitor C11, resistors R11 and R21, a first switch unit D11, a second switch unit Q11, a switch node A1, a power source node B1+, a ground node F1, and a transform element TF1. The first switch unit D11 is a diode for being activated when a voltage level at an input terminal of the first switch element D11 is higher than a voltage level at an output terminal of the first switch element D11. The second switch unit Q11 is a metal-oxide semiconductor transistor for adjusting a resistance of a resistor between the drain and the source of the second switch unit Q11 according to a voltage level at the gate of the second switch unit Q11 for enabling or disabling an electrical connection between said drain and said source of the second switch unit Q11. The transform element TF1 is a coupling element of double winding, for performing power transformation on received current and for outputting power at a specific voltage level. When the second switch unit Q11 is short-circuited, a received current of the power source node B1+ flows through the transform element TF1, the second switch unit Q11, the resistor R21, and the ground node F1 in order. At the same time, since an input voltage level of the first switch unit D11 is not higher than an output voltage level of the first switch unit D11, the first switch unit D11 remains open-circuited. When the second switch unit Q11 is open-circuited, a current flow from the drain to the source of the second switch unit 11 dissipates, and then the transform element TF1 resists said dissipating current flow for releasing power. Therefore, a current flow leads to the first switch unit D11 for activating the first switch D11, and then heads back to the power source node B1+ through a parallel connection of both the capacitor C11 and the resistor R11. Moreover, when the second switch unit Q11 becomes open-circuited instead of short-circuited, a counter-electromotive force generated from the transform element TF1 charges the capacitor C11 with a spike voltage. After the capacitor C11 absorbs power from the spike voltage, said power is discharged from the resistor R11, and additional power consumption is thus generated.
According to variant voltages received by the power source node B1+, the conventional power transform device 100 may be operated under two different modes including a high-load mode and a low-load mode. For meeting regulations of power, power consumption of the conventional transform device 100 under the low-load mode has to be less than 1 watt. Therefore, a capacitance of the capacitor C11 has to be decreased whereas a resistance of the resistor R11 has to be increased. Similarly, for reducing additional power consumption, an operating frequency of the conventional power transform device 100 has to be lowered for operating under a burst mode. However, when the operating frequency of the conventional power transform device 100 is lowered to an audio frequency domain, harsh noise is generated. The harsh noise may be relieved by reducing power absorbed with the series connection formed from both the capacitor C11 and the resistor R11. However, when the conventional power transform device 100 operates under the high-load mode, since power absorbed by the series connection of the capacitor C11 and the resistor R11 is getting lower, the spike voltage cannot be absorbed completely, and related electromagnetic interference (EMI) is getting heavier also. On the contrary, when the spike voltage is effectively absorbed with higher power absorbed by the series connection of the capacitor C11 and the resistor R11, related noise cannot be relieved.